Since the arrival on the market of the USB (Universal Serial Bus) standard to connect computers and all kinds of peripheral devices, more and more electronic devices that are meant to interact with computers are given the form factor of USB sticks, i.e., a highly portable somewhat flattened and elongated shape with a USB connector at one end such that the device can be easily carried around and can be easily plugged into a USB connector of a host computer. Examples of such USB devices include USB memory sticks, USB authentication tokens, wireless modems with a USB connector, MP3 players with a USB connector, and so forth.
USB devices with the form factor of a USB stick often come with a cap that can be plugged on the device's body to hide and protect the USB connector. These caps offer a number of advantages. On the one hand such a cap protects a USB device's USB connector from external influences (such as dirt or splash water or scratching metal objects such as keys) that could damage the connector or could be detrimental to the correct functioning of the connector. On the other hand such a cap protects the textile of a user's clothing against the wear and tear that the contact with the hard and relatively sharp metal edges of the USB connector might cause when the USB device is carried in a user's pockets.
The mechanism that ensures that a cap remains attached to a USB device's body when that cap is plugged on the body to protect the device's USB connector should satisfy a number of requirements. Two of these requirements are not obvious to reconcile. On the one hand when the cap is plugged on the device's body it should hold to the device firmly enough to minimize the likelihood that the cap is unintentionally detached, even if the cap and device are submitted to forces that would tend to pull them apart (for example the force of gravity in case the user is carrying the device hanging on a lanyard that is attached to either the device's body or the device's cap). On the other hand it should be easy enough for a user to pull off the cap from the device without having to carry out complicated or time-consuming actions or having to exert an uncomfortable level of force. Another requirement is that the cap's attachment mechanism should remain effective even when the cap has been plugged on and off many times.
One solution is to tune the dimensions of the device's body and the cap such that at the contact area between the cap and the device's body when the cap is plugged on the body, the cap is slightly smaller or narrower than the body, which, in combination with a certain elasticity of cap and/or body, results in a pressure force between cap and body at the contact area. By ensuring that the contact area is more or less parallel to the axis along which the cap is plugged onto the body, the friction force which results from the pressure force and which is parallel to the contact area will counteract forces that would tend to pull off the cap. This solution, while popular, has however a number of disadvantages. The dimensions of the cap and the body should be finely tuned to each other within narrow margins. If the cap is too wide with respect to the body it will not sufficiently hold on to the body when plugged on. Conversely, if the cap is too narrow with respect to the body, the user may have to exert excessive force to plug the cap on or to pull it off. Ensuring this narrow fine tuning of the dimensions of cap and body may sometimes be problematic. This is in particular true for USB devices consisting of a substrate onto which the device's electronics have been arranged, enclosed in a combination of two (or more) shells which are attached to each other, and a protruding USB connector that comes out of the enclosure. Often the device's Printed Circuit Board (PCB) with electronic components is housed inside a housing which consists of two shells that are placed each on one side of the PCB with the direction of mounting the shells being perpendicular to the plane of the PCB so that the shells can fix the PCB well. Depending on the technique to attach these shells, fine tuning the exact dimension of the resulting enclosure within narrow margins can be difficult or even impossible. For example, a popular technique to manufacture a plastic housing for USB tokens is to ultrasonically weld together an upper and a lower plastic shell. Ultrasonic welding is an interesting technology for assembling security devices such as USB tokens since it can be difficult to open an ultrasonically welded device without damaging the housing and leaving visible traces. This contributes to the tamper evidence of the device. However, it can be difficult to control the process of ultrasonic welding to a sufficient degree to ensure that the overall thickness of the resulting housing can be defined within a sufficiently narrow margin. With ultrasonic welding, the resulting position of the two shells relative to each other can typically vary with as much as 0.25 mm. Another disadvantage is that the wear and tear of repeatedly plugging and removing the cap may eventually cause some deformation of the cap and/or the device's body such that the connection between cap and body eventually becomes looser so that the fastening of the cap to the body becomes less reliable.
Another solution to removably fasten a cap to the body of a USB device is to provide both cap and device with a compatible screw thread and screw the cap on the body. This solution has however the following disadvantages. This solution may require that the cap and the body have a cylindrical form (at least at the contact area) while most USB devices tend to be considerably broader than thick (for example to be as compact as possible while still remaining compliant with the USB specification). Also the action of screwing the cap on or off the body may be perceived by users as being too time-consuming in comparison to just simply plugging a cap on the device's body or pulling it off the device's body.
What is therefore needed is a mechanism to fasten a cap to the body of a USB device that does not have the disadvantages of the solutions described above.